As logic integrated circuits have migrated to lower working voltages in the surge for higher operating frequencies and as overall system sizes have continued to decrease, power supply designs with smaller and higher efficiency power modules are in demand. In an effort to improve the efficiencies and increase power densities, synchronous rectification has become necessary for these types of applications. Synchronous rectification has gained great popularity in the last ten years as low voltage semiconductor devices have advanced to make this a viable technology.
Synchronous rectification refers to using active devices such as the MOSFET as a replacement for shocky diodes as rectifier elements in circuits. Recently, self-driven synchronous schemes have been widely adopted in the industry as the desired method for driving the synchronous rectifiers in DC-to-DC modules for output voltages of five volts and below. Most of these self-driven schemes are designed to be used with a very particular set of topologies commonly known as "D,1-D" (complimentary driven) type topologies. In these types of converters, the power transformer signal in the secondary winding has a correct shape and timing to directly drive the synchronous rectifiers with minimum modifications.
In topologies such as the hard switched Half Bridge (HB) and Full Bridge (FB) rectifiers and in push-pull topologies, the transformer voltage has a recognizable zero voltage interval making it undesirable to implement self-driven synchronous rectification. Using the transformer voltage to drive the synchronous rectifiers results in conduction of the parasitic anti-parallel diode of the MOSFETs used for the synchronous rectifiers for a significant portion of the free willing interval, negatively effecting the efficiency of the module, which is undesired. As a result, it is necessary to use an external drive circuit with these circuit topologies. In these implementations, the resonant reset interval has been adjusted to provide the correct gate drive signal during the free willing interval. Therefore, the externally driven implementation offers a better solution for synchronous rectification in many instances. However, the prior art externally driven synchronous rectification is both complex and costly.
Traditionally, the external driving circuit of a bridge-type synchronous rectification DC-to-DC converter includes a center tapped gate drive, an integrated circuit which inverts the timing signal, as necessary, and drives the synchronous rectifiers, and a pair of totem pole drivers. The center tap of such a driving circuit results in an extra terminal which increases the size of the transformer and also increases cost. The integrated circuit needed for the driver also increases cost and lessens the reliability of the circuit as more parts are needed for driving the synchronous rectifier circuit. Thus, what is needed is a simplified external driving circuit for bridge-type synchronous rectification which is both physically smaller and less costly.